1. Field of the Invention
This invention relates generally to digital data processing apparatus. In particular, this invention relates to a process control apparatus for reading the condition of a selected variable and for generating a next instruction to be implemented based on the observed condition of the selected variable.
2. Description of the Prior Art
C. Y. Lee in his paper "Representations of Switching circuits by Binary Decision Programs" published in the Bell System Technical Journal of July, 1959, pages 985 to 999 described a binary decision program wherein the choice of the next instruction depends on the binary value of the input variable. For this purpose Lee created the following computer instruction: EQU T x; A,B
where
T=Test PA1 x=Boolean variable PA1 A,B=transfer addresses
Lee described the operation of the instruction as "if x is a logic zero then proceed to address A. Otherwise, go to address B." Lee then proceeded to show that a program can be composed of a sequence of instructions consisting only of conditional transfers and terminating in an output and that this binary decisions program can be created to solve any Boolean function.
Raymond T. G. Boute in his paper "The Binary-Decision Machine as a Programmable Controller", Euromico Newsletter, Vol 1, No. 2, 1976, pp. 16-22 described a binary decision apparatus designed to execute binary decision programs. Boute's binary decision machine is also the subject of U.S. Pat. No. 4,393,469 to Raymond T. G. Boute which issued Jul. 12, 1983. There is described therein a process control apparatus which reads the condition of a selected variable and generates a next instruction to be implemented based on the condition of the selected variable. The next instruction is that of reading one of two possible instructions, one of which causes the state of an output variable to be read and/or set and the other calls for reading the value of another selected input. In this manner, a program cycle is completed so that the values of all output variables can be set and read.
While Boute's process control apparatus was a significant departure from processor apparatus of the time and functions adequately to evaluate Boolean control problems as quickly as possible, Boute's process control apparatus has certain limitations which prevented this binary decision controller from being a fast and efficient means of controlling portions of entire robotic systems. Specifically, the invention of U.S. Pat. No. 4,393,469 does not provide for a system reset which would reset the components thereof to a known state and may have timing problems that are correctable by using external hardware. In addition, the invention of U.S. Pat. No. 4,393,469 assumes that the input variables are stable during the evaluation process, allows only a single bit to be output as opposed to a word, does not provide for the use of external event indicators and for the capability of using subroutines. Further, the invention of U.S. Pat. No. 4,393,469 does not provide for the need to count which is required by the majority of control problems such as the displacement of a stepping motor and does not provide for serial communications with external devices.